In modern internal combustion engines, lambda sensors for determining the composition of the exhaust gas and for controlling the internal combustion engine are used to optimize the pollutant emission and the exhaust gas aftertreatment. Lambda sensors determine the oxygen content of the exhaust gas, which is used to regulate the air-fuel mixture supplied to the internal combustion engine and therefore the exhaust gas lambda upstream from a catalytic converter. The air and fuel supply of the internal combustion engine are regulated via a lambda control loop in such a way that an optimum composition of the exhaust gas is achieved for the exhaust gas aftertreatment by the catalytic converters provided in the exhaust duct of the internal combustion engine. In gasoline engines, lambda is generally regulated to 1, i.e., a stoichiometric ratio of air to fuel. The pollutant emission of the internal combustion engine may thus be minimized.
Various forms of lambda sensors are used. In a two-point lambda sensor, which is also referred to as a jump sensor or Nernst sensor, the voltage-lambda characteristic curve has a sudden drop at lambda=1. It therefore essentially permits the differentiation between rich exhaust gas (λ<1) during operation of the internal combustion engine with excess fuel and lean exhaust gas (λ>1) during operation with excess air and enables a regulation of the exhaust gas to a lambda of 1.
A broadband lambda sensor, also referred to as a continuous or linear lambda sensor, enables the measurement of the lambda value in the exhaust gas in a broad range around lambda=1. Therefore, for example, an internal combustion engine may also be regulated to a lean operation with excess air.
A continuous lambda regulation upstream from the catalytic converter is also possible by way of a linearization of the sensor characteristic curve using a more cost-effective two-point lambda sensor in a restricted lambda range. The requirement for this purpose is that an unambiguous relationship exists between the sensor voltage of the two-point lambda sensor and lambda. This relationship must exist over the entire service life of the two-point lambda sensor, since otherwise the precision of the regulation is inadequate and impermissibly high emissions may occur. This condition is not met due to manufacturing tolerances and aging effects of the two-point lambda sensor.
To carry out a continuous lambda regulation using a two-point lambda sensor, determining and compensating for a voltage offset of the existing voltage-lambda characteristic curve in relation to a reference voltage-lambda characteristic curve of the two-point lambda sensor, which is constant over the entire lambda range, by an adjustment of the sensor voltage during overrun fuel cutoff of the internal combustion engine, in which no fuel is supplied to the internal combustion engine, is known. Building thereon, the publication DE 10 2010 027 984 A1 describes a method for operating an exhaust system of an internal combustion engine, in which at least one parameter of the exhaust gas flowing in an exhaust duct is detected by an exhaust sensor. It is provided that during an operating state of the internal combustion engine in which injection and combustion of fuel do not occur, fresh air is supplied to the exhaust duct upstream from the exhaust sensor with the aid of a fresh air supply associated with the exhaust system, and the exhaust sensor is adjusted during this and/or thereafter.
Sufficiently good compensation of the voltage offset is only possible, however, if it is equally strongly pronounced not only in the event of overrun fuel cutoff with correspondingly oxygenated exhaust gas, but rather in the entire lambda range. This may be the case if the voltage offset has a single cause. However, there are typically multiple superimposed causes for a deviation of the voltage-lambda characteristic curve in relation to the reference voltage-lambda characteristic curve. These may be pronounced at different strengths in various lambda ranges, whereby the voltage offset changes as a function of the exhaust gas lambda. In particular, the causes may be pronounced at different strengths in the lean lambda range and in the rich lambda range. Such a voltage offset dependent on the lambda cannot be sufficiently compensated for by an adjustment in the event of overrun fuel cutoff. A further disadvantage of the method is that modern engine concepts have fewer and fewer overrun phases, which restricts the possibility of such an overrun adjustment.
Therefore, two-point lambda sensors are usually used upstream from the catalytic converter with a two-point regulation. This has the disadvantage that in operating modes for which a lean or rich air-fuel mixture is necessary, for example, for catalytic converter diagnosis or for component protection, the target lambda may only be set by pilot control, but may not be regulated.